1. Field of the Invention
This invention relates to an improved wheel speed sensor and, more specifically, to such a sensor installed at the end of an axle spindle within the wheel hubcap area and utilized in an anti-skid braking system.
2. Description of the Prior Art
It has been found desirable in the operation of an anti-skid braking system to provide an input representing the wheel speed. The wheel speed input is usually in the form of electrical signals indicative of the angular velocity of the wheel. These signals are generally derived from an electromagnetic sensor, including a magnet carried by a stationary portion of the trailer such as the axle spindle and a toothed or notched metal ring which rotates with the wheel relative the magnet. As the ring teeth pass the magnetic sensor, the resulting variations in flux produce a signal, the frequency of which is a function of the angular velocity of the wheel.
Because of the free-wheeling aspect of trailer wheels and the fixed mounting of the trailer axles upon which they are rotatably mounted, most trailer wheel speed sensors heretofore provided have been rigidly mounted within the open end of the spindle of the axle so that the sensor connecting wire can extend axially within the hollow interior of the axle. The wire exits the interior of the axle at an intermediate region remote from the wheel to be connected to the anti-skid braking system. Since the sensor extends from the end of the spindle, the notched ring is accordingly mounted within the hubcap rather than within the wheel housing so that only the hubcap need be removed to allow access to the sensor.
During installation of the wheel on the spindle, the hubcap is filled with lubricating fluid for the wheel bearing. It is obvious that with the hubcap speed sensor having a wire passing through an open end of the spindle, lubricating fluid would escape from the hubcap to the interior of the axle. Because of this problem, it has generally been the practice in the prior art to mount the speed sensor at and against the interior surface of the open end where sealing could be effectively provided. However, since the operating gap between the sensor and the ring must be a predetermined value on the order of about 0.030 inches and constantly maintained throughout wheel rotation, the configuration utilized in these prior art devices required the interior of the open end to be precision machined. Insuring the manufacturing tolerances of the spindle and hubcap are defined and maintained in this manner has been found to add significantly to the manufacturing cost of the spindle. Further, it has been the practice to precision machined openings with a predetermined inside diameter to match the particular sensor design employed. Accordingly, the spindle is then unable to accommodate other sensors of this general type which have different diameter dimensions.
Wheel speed sensors for braking control systems or the like are also utilized at the wheels of the steering axle but have generally been located within the brake area. U.S. Pat. Nos. 3,772,548 and 3,774,061 disclose sensors of this type which can be seen to require an annular-type rotor. A feature tending to complicate maintenance is the need to remove the entire wheel assembly to gain access to sensors of this type. Since the spindle of a steering axle does not include an open end and the overall diameter of the steering spindle is less than that of a spindle on the fixed axle, there appears to have been no attempts made to mount the prior art hubcap sensor devices discussed hereinabove on a steering axle.
It is therefore of concern that a wide variety of wheel speed sensoring devices have been utilized which have increased the cost of manufacturing the axle assembly, which have lacked compatibility with other axle assemblies not specifically adapted for their use, and/or which have generally increased maintenance and inventory problems.